1. Field of the Invention
The present invention relates generally to turbidity sensors and, more particularly, to a turbidity sensor which is configured to decrease the adverse affect that can result from air bubbles in a stream of fluid whose turbidity is to be determined and, in addition, is provided with the capability of determining the rate of change of turbidity as a function of time to enable an appliance to make various energy and time saving decisions.
2. Description of the Prior Art
Turbidity has been defined, by the American Public Health Association, as an expression of the optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through a fluid. Therefore, turbidity is generally measured as the ratio of scattered light to directly transmitted light. This can be interpreted as a measure of the relative clarity of a liquid. It should be understood that turbidity is not a direct measurement of suspended particles in fluid but, instead, is a measurement of the scattering effect that suspended particles have on light. The amount and color of light that is scattered by particles suspended in a fluid is dependent on the size, shape, composition and refractive index of the particles.
A light beam will pass through pure water while remaining relatively undisturbed. If solids are suspended in the water, they interfere with the light transmittance through the water in a way which is related in a way to the type of particle and the wavelength of the incident light. A minute particle interacts with the light by absorbing the energy of the light and then reradiating the light energy in all directions. This scattered light depends on the ratio of particle size to the wavelength of the incident light.
During the first few years of this century, early attempts to quantify turbidity were made. A standard suspension fluid using 1000 ppm of diatomaceous earth in distilled water was used and dilutions of the reference suspension fluid resulted in a series of standard suspensions that were used to derive a ppm silica scale for calibrating the contemporary turbidimeters. These devices typically consisted of a flat bottomed glass tube and a special candle. Measurements were made by slowly pouring a turbid sample into the tube until the visual image of the candle, when viewed from the open top of the tube, defused to a uniform glow. This was called the extinction point and, although this method was very subjective, trained workers achieved remarkably consistent results. Modern turbidity sensors utilize a light source and one or more light sensitive components. Typically, the light source and light sensitive components are disposed around the outer periphery of a transparent conduit through which a fluid is caused to flow. One light sensitive component is placed directly across the conduit from the light source on a common diameter. The other light sensitive component is disposed at a 90.degree. degree offset from both the light source and the other light sensitive component. The first light sensitive component, which is placed opposite to the light source on a common diameter, is intended to receive light emanated directly from the light source. The second light sensitive component is intended to receive light scattered by particulate matter within the fluid flowing through the conduit. A ratio of the intensities of light received by the two light sensitive components can be used as a measurement of the fluid's turbidity. Many different types of fluidity sensors are known to those skilled in the art.
U.S. Pat. No. 5,140,168, which issued to King on Aug. 18, 1992, discloses a turbidimeter signal processing circuit that uses alternating light sources. The turbidimeter includes a housing having a cavity with an inlet through which a fluid flows. Two emitters are alternately driven by an alternating signal having a given frequency to transmit modulated light beams through the fluid. Two detectors produce signals representing the intensity of scattered and unscattered light within the fluid. Each of these detector signals is processed to measure the level of the signal component at the given frequency. Such processing includes filtering and phase demodulating the detector signals to produce a signal indicative of the levels of the component signals at the given frequency. The turbidity is calculated from the signal levels measured as each emitter is excited.
U.S. Pat. No. 5,104,228, which issued to Baillie on Apr. 14, 1992, describes a photosensitive turbidimeter with a nonfouling measurement chamber. The turbidity of liquids, including oil contaminated water, may be measured by an apparatus having a first elongated shell member defining a turbid liquid measurement chamber and opposed head members forming windows for transmitting a light beam through the measurement chamber between a light source and a photosensitive element. A second shell member is disposed around the first shell member and defines with the first shell member and the opposed head members clear liquid supply chambers for supplying a clear liquid to wash over the windows and prevent contact of the turbid liquid with windows during operation of the apparatus. A flow of clear liquid, such as water, may be controlled by a pump and a throttling valve with orifices in each flow line to limit the flow of clear liquid. A mixture of clear liquid and turbid liquid is discharged from the turbidity measurement chamber into a discharge manifold formed between the shell members.
U.S. Pat. No. 5,099,123, which issued to Harjunmaa on Mar. 24, 1992, describes a method for determining by absorption of radiations the concentration of substances in absorbing and turbidmatricies. The method and apparatus for noninvasively testing analytical substances in turbid matrices comprises a sample which is irradiated with a beam of electromagnetic energy at two alternating wavelengths at which the absorption by the background is the same at one of which the radiation is absorbed by the analyte and at the other is not. The apparatus comprises means which enable the control of an input energy at the two wavelengths so that at the output from the sample the electrical signals issuing after detection cancel in the absence of the analyte in the sample. When analyte is present cancellation no longer occurs and a signal proportional to the analyte concentration in the sample is produced. The apparatus is also designed for shifting the response to zero when a calibrating known concentration of analyte is used as a standard, thus providing a controllable zeroing base line.
U.S. Pat. No. 5,059,811, which issued to King et al on Oct. 22, 1991, discloses a turbidimeter having a baffle assembly for removing entrained gas. The turbidimeter includes a housing with a cavity having an inlet through which the fluid enters the bottom of the cavity and an outlet through which the fluid exits the top of the cavity. A removable baffle assembly is located within the cavity between the inlet and the outlet. The baffle assembly is formed by three vertical plates which are spaced from each other and extending across substantially the entire cross sectional area of the cavity. The first plate defines a first passage near the top of the cavity through which all of the fluid entering the cavity must flow. The second and third walls define a second passage near the top of the cavity through which gas bubbles entrained in the fluid travel to the outlet. A third passage is defined between the first wall and the outlet and a mechanism is provided for measuring the turbidity of the fluid flowing through the third passage. A calibration device formed by a block of glass ceramic material is insertable in the third passage to simulate of known turbidity fluid.
U.S. Pat. No. 5,048,139, which issued to Matsumi on Sep. 17, 1991, discloses a washing machine with a turbidimeter and a method of operating the turbidimeter. The washing machine uses a turbidimeter to measure turbidity of cleaning water for controlling the duration of its washing and cleaning cycles. Quality of this control is improved by taking measurements when the water flow is weak, so that the effects of foams are negligible, and waiting until turbidity drops at the beginning of the cycle to detect the initial value used in subsequent steps. Sensitivity of the turbidimeter is automatically adjusted for accuracy when the operation is temporarily stopped and restarted during a cycle.
U.S. Pat. No. 4,999,514, which issued to Silveston on Mar. 12, 1991, discloses a turbidity meter parameter selection and waiting. The meter has a sensor unit supported in a fluid under test with a light source and at least two light sensors supported so that one light sensor is in line with the source to receive transmitted light and the remaining sensor or sensors are arranged to receive light scattered by the fluid. Both the source and sensors have flow forming chambers connected to a source of pressurized fluid so that a thin layer of this fluid is caused to flow over lenses of the source and sensors to prevent deposition of material from the fluid under test. The signals from the sensors are digitized and the intensity of the source is digitally controlled to maintain at least one of the sensor signals within a suitable range, thus enabling operation over a wide range of turbidities and automatic selection of turbidimetric and nephelometric modes of operation as appropriate.
U.S. Pat. No. 4,906,101, which issued to Lin et al on Mar. 6, 1990, describes a turbidity measuring device for measuring turbidity in static or dynamic streams wherein the fluid has up to 8500 ppm solids and at depth of up to 8 inches. The device contains a high intensity light source, means for controlling the wavelength of the transmitted light to between 550 and 900 nanometers to filter out color variables in the streams. It also contains a very sensitive photosensor aligned with the viewing means for picking up the light transmitted through the streams.
Turbidity sensors can be used in modern home appliances, such as dishwashers, to monitor the progress of cleaning cycles. The turbidity of the water in a dishwashing machine or clothes washing machine can provide an important indication of the efficacy of the cleansing process. It has been determined that the use of a turbidity sensor can be adversely affected by the presence of air bubbles in the fluid stream being monitored. If, for example, an air bubble lodges at a position on the inner cylindrical surface of a conduit proximate a light sensitive component, the bubble could block the passage of light to the light sensitive component and seriously distort the sensor's ability to accurately determine the turbidity of the fluid flowing through the conduit.